| author | timkelly |
| Thu, 10 Dec 2009 13:45:47 -0600 | |
| branch | RCL_2_4 |
| changeset 671 | 80524b72f957 |
| parent 0 | fb279309251b |
| child 990 | 5d016a880824 |
| child 1024 | 48b401835d0a |
| permissions | -rw-r--r-- |
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" |
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"http://www.w3.org/TR/html4/loose.dtd"> |
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<html><head> |
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<title>Virtual Memory and Run Addresses</title> |
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<link href="sysdoc-eclipse.css" type="text/css" rel="stylesheet" media="screen"> |
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<link href="sysdoc-eclipse.css" type="text/css" rel="stylesheet" media="print"> |
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<link href="../../book.css" type="text/css" rel="stylesheet" > |
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<div class="Head1"> |
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<h2>Virtual Memory and Run Addresses</h2> |
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</div> |
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<div> |
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<p> |
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Symbian OS devices have an MMU which is used to map the addresses seen |
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by running code to real addresses of memory and I/O. The MMU in effect creates |
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a virtual memory map, allowing scattered blocks of RAM to appear contiguous, or |
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for a section of memory to appear at different addresses in different |
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processes, or not at all. |
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</p> |
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<p> |
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Symbian OS uses the MMU to provide memory protection between processes, |
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to allow sharing of memory, efficient allocation of RAM and to make all |
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processes “see” the same memory layout. Three different memory models are |
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supported by Symbian OS on ARM CPUs: |
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</p> |
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<ul> |
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<li> |
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<p> |
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moving model: this is the model familiar from EKA1 where processes |
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are moved to a run-address in low memory when executing and moved back to a |
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home-address in high memory when not running. |
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</p> |
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</li> |
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<li> |
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<p> |
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direct model: this is used when the CPU does not have an MMU, or is |
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emulating a system without an MMU. Not normally used, but occasionally useful |
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for development boards |
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</p> |
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</li> |
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<li> |
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<p> |
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multiple model: only supported in ARM architecture V6 and above, |
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each process has its own set of MMU tables. A context switch changes the |
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current MMU table to the new thread’s table, instead of moving memory about in |
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a single table as with moving model. |
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</p> |
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</li> |
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</ul> |
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</div> |
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<div class="Head3"> |
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<h3><span class="Bodytext"><a name="1.10.6"></a></span>Fixed processes</h3> |
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</div><div> |
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<p> |
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For ARM architectures with a virtually-tagged cache, fixed processes |
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avoid the need to flush the cache on context switches by keeping all the code |
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and data at a fixed address. This implies that there can only ever be one |
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instance of each fixed process because the data chunk address cannot be |
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changed. |
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</p> |
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<p> |
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Important servers such as the file server and window server are |
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fixed. |
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</p> |
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<p> |
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There is no limit to the number of fixed processes that can be |
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supported. The kernel will attempt to use ARM domains for fixed process |
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protection, but there are a limited number of domains so when they are |
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exhausted normal MMU techniques will be used. Domains are slightly faster in a |
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context switch but this is negligible compared to the real purpose of the fixed |
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process in avoiding the cache flush. |
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</p> |
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</div> |
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<div id="footer">Copyright © 2009 Nokia Corporation and/or its subsidiary(-ies). All rights reserved. <br>License: <a href="http://www.eclipse.org/legal/epl-v10.html">http://www.eclipse.org/legal/epl-v10.html</a></div> |
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</body> |
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</html> |
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